1. Field of the Invention
The present invention relates to a scroll pump which includes plate scrolls having nested scroll blades, and a tip seal(s) that provides a seal between the tip of the scroll blade of one of the plate scrolls and the plate of the other plate scroll.
2. Description of the Related Art
A scroll pump is a type of pump that includes a stationary plate scroll having a spiral stationary scroll blade, and an orbiting plate scroll having a spiral orbiting scroll blade. The stationary and orbiting scroll blades are nested with a clearance and predetermined relative angular positioning such that a pocket (or pockets) is delimited by and between the scroll blades. The scroll pump also has a frame to which the stationary plate scroll is fixed and an eccentric drive mechanism supported by the frame. These parts generally make up an assembly that may be referred to as a pump head (assembly) of the scroll pump.
The orbiting plate scroll and hence, the orbiting scroll blade, is coupled to and driven by the eccentric driving mechanism so as to orbit about a longitudinal axis of the pump passing through the axial center of the stationary scroll blade. The volume of the pocket(s) delimited by the scroll blades of the pump is varied as the orbiting scroll blade moves relative to the stationary scroll blade. The orbiting motion of the orbiting scroll blade also causes the pocket(s) to move within the pump head assembly such that the pocket(s) is selectively placed in open communication with an inlet and outlet of the scroll pump.
In an example of such a scroll pump, the motion of the orbiting scroll blade relative to the stationary scroll blade causes a pocket sealed off from the outlet of the pump and in open communication with the inlet of the pump to expand. Accordingly, fluid is drawn into the pocket through the inlet. Then the pocket is moved to a position at which it is sealed off from the inlet of the pump and is in open communication with the outlet of the pump, and at the same time the pocket is collapsed. Thus, the fluid in the pocket is compressed and thereby discharged through the outlet of the pump. The sidewall surfaces of the stationary orbiting scroll blades need not contact each other to form a satisfactory pocket(s). Rather, a minute clearance may be maintained between the sidewall surfaces at the ends of the pocket(s).
A scroll pump as described above may be of a vacuum type, in which case the inlet of the pump is connected to a chamber that is to be evacuated.
Furthermore, oil may be used to create a seal between the stationary and orbiting plate scroll blades, i.e., to form a seal(s) that delimits the pocket(s) with the scroll blades. On the other hand, certain types of scroll pumps, referred to as “dry” scroll pumps, avoid the use of oil because oil may contaminate the fluid being worked by the pump. Instead of oil, dry scroll pumps employ a tip seal or seals each seated in a groove extending in and along the length of the tip (axial end) of a respective one of the scroll blades (the groove thus also having the form of a spiral). More specifically, each tip seal is provided between the tip of the scroll blade of a respective one of the plate scrolls and the plate of the other of the plate scrolls, to create a seal which maintains the pocket(s) between the stationary and orbiting scroll blades. Further in this respect, scroll pumps of the type described above typically require a certain degree of axial compliance among respective parts of the pump head assembly to maintain an effective seal between the opposing scroll blades and plates.
In general, there are two types of tip seal arrangements to meet these requirements: energized and non-energized. An energized type of tip seal arrangement includes a tip seal seated in the tip of the scroll blade of one of the plate scrolls, and a spring that biases the tip seal against the plate of the other of the plate scrolls. A typical non-energized type of tip seal arrangement has only a solid plastic tip seal seated in the tip of the scroll blade of one of the plate scrolls and the solid plastic tip seal directly confronts the plate of the other of the plate scrolls.
In the spring-biased tip seal arrangements, the friction produced by the engagement of the tip seal with the opposing scroll plate is limited in that it does not exceed a value corresponding to the maximum force that can be exerted by the spring on the tip seal. However, spring-biased tip seals are continuously worn because they are constantly biased into engagement with the opposing scroll plate. As a result, spring-biased tips seals must be replaced rather frequently. The solid plastic tip seals of the non-energized arrangements have a relatively longer useful life than the conventional spring-biased tip seals. However, the use of solid tip seals presents its own set of problems.
For instance, the tolerances of axial dimensions of various components of scroll pumps that employ non-energized tip seals must be maintained within narrow ranges to ensure that the tips seals are properly positioned in the pump head. More specifically, precise axial positioning ensures that any gap between a solid tip seal and the opposing scroll plate is minimal. If, the gap is too large, the tip seal will not produce an effective seal with the opposing scroll plate. However, if the tip seal is compressed too much between the scroll blade and the opposing scroll plate, the resulting friction and heat can overload and damage not only the seal itself but also parts of the pump such as the bearings of the drive mechanism.